[9.18] chg: ci: Ensure changelog job builds docs with the new entry

The changelog job is supposed to test that the text from GitLab MR
title&description is valid rst syntax and can be built with sphinx. In
49128fc1, the way gitchangelog generates entries was changed - it no
longer writes to the changelog file, but generates output on stdout
instead. Ensure the generated notes is actually written to (some)
rendered file which is part of the docs so that the subsequent sphinx
build attempts to render the note.

Backport of MR !9804

Merge branch 'backport-nicki/ci-fix-changelog-job-9.18' into 'bind-9.18'

See merge request isc-projects/bind9!9998